Controllable degradation of alpha-olefin polymers using irradiation



United States Patent Ofilice 3,349,018 CONTROLLABLE DEGRADATION OFa-OLEFIN POLYMERS USING IRRADIATION James E. Potts, Millington, N.J.,assignor to Union Carbide Corporation, a corporation of New York NoDrawing. Filed Oct. 28, 1963, Ser. No. 319,562 10 Claims. (Cl. 204-1592)This invention relates to a-olefin polymers and to a method forirradiating said polymers to control the molecular weight or molecularweight distribution thereof. More particularly, this invention relatesto a method for controllably degrading a-olefin polymers such aspolypropylene without the use of heat and/or mechanical shear and to theimproved a-olefin polymers produced thereby.

Normally solid a-olefin polymers and especially propylene polymers arerecognized in the plastics industry as possessing great commercialpotential because of the many advantages provided by said polymers overother polyolefin polymers such as polyethylene. Propylene polymers, forexample, can be produced in amorphous or crystalline form depending uponthe catalysts and the reaction conditions employed. The high molecularweight, highly crystalline propylene polymers are characterized by theirclarity, their high toughness and strength, their excellent mechanicalresiliency and their stiflness moduli.

Many of the commerical processes presently employed in the preparationof a-olefin polymers result in po ymers which are of extremely highmolecular weight and highly isotactic. Propylene polymers havingintrinsic viscosities as high as 15 have been obtained; however, thesepolymers are so high in molecular weight that they are essentiallyintractable and cannot be molded or extruded. Heretofore, the molecularweight of wolefin polymers was commonly controlled during polymerizationby addition of various chain terminators such as hydrogen. The use ofchain terminators is not completely satisfactory, however, because ofproductivity losses which result from chain termination as well as thed'fticulties which arise in accurately feeding small amounts ofterminators to the reaction medium. In addition, polymer formation andrecovery techniques vary with the molecuar weight of the polymer thus,optimum performance of the production unit cannot be achieved with aWide product mix.

Other post polymerization processes such as pyrolysis and/r mechanicalshear are also employed to control the molecular weight or degrade orreduce the molecular Weight of the a-olefin polymers.

Pyrolysis of propylene polymers for example is not completelysatisfactory because of the ease with which such polymers undergooxidative degradation at elevated temperatures thereby requiring thepresence of antioxidants. The high temperatures involved in thermalpyrolysis however, tend to decompose the antioxidants producing color,odor and chemical compounds of questionable merit from a toxicityviewpoint. Moreover, certain stabilizers are completely destroyed bypyrolysis temperatures and must be added in a subsequent step.

Many attempts to degrade high molecular weight a-olefin polymers bypyrolytic means have been unsuccessful due to the absence of melt flowproperties above the crystalline melting point. Pyrolysis techniques arealso subject to heat transfer problems with the mass of molten polymerresulting in varying degrees of degradation in different parts of theresin mass. Mechanical shear techniques are also subject to variable andlocalized degradation since it is extremely diflicult to shear allportions of a resin mass between metal surfaces without producingextensive degradation of some parts of the resin.

erably at least percent Accordingly, it is an object of the presentinvention to provide a method for controllably reducing the molecularweight or degrading a-olefin polymers to any desired molecular weightlevel without the use of pyrolysis and/ or mechanical shear.

It is another object of the present invention to impart unique meltrheological properties to a-olefin polymers thereby providing improvedthermoforming properties than heretofore obtained with a-olefin polymersdegraded to the same melt flow by pyrolysis or mechanical shear.

It is still another object of this invention to provide a method forsubstantially reducing the draw resonance phenomenon which occurs in thehigh speed extrusion of polymers.

These and other objects are accomplished in accordance with the presentinvention which provides a method for uniformly and controllablydegrading wolefin polymers which comprises subjecting the z-olefinpolymer in air to ionizing radiation having an energy level in the rangeof from about 50 kev. (kilo electron volts) to about 20 mev. (millionelectron volts) to a total dose between about 0.01 to about 3 megarepsbut less than that which causes gelation.

The u-olefin polymers which can be suitably employed in the presentinvention are those which are normally solid at room temperature and arerepresented by the repeating unit wherein R is an alkyl group containingfrom about 1 to 18 carbon atoms. Illustrative of such (it-olefinpolymers are polypropylene, poly(butene-l), p0ly(pentene-l),poly(4-methylpentene-1), poly(hexene-l), poly(octene l),poly(octadecene-1), and the like. It is considered preferable in thisinvention that the repeating unit of the OC-Olefin polymers employedpossess ratios of secondary (methylene) carbon atoms to tertiary(methylidyne) carbon atoms in the range of 1:1 to 16:1 and mostpreferably 1:1 to 6:1. It has been found that cross-linking becomes thepredominant effect of irradiation above a methylene to methylidynecarbon atom ratio of about 16:1 which corresponds to a repeating unitcontaining 18 carbon atoms as in poly(octadecene-l). It is believed thatthe cessation of the predominance of degradation and the onset ofcross-linking is due to the higher percentage by weight of methylenelinkages present at the higher ratios which is more nearlycharacteristic of a polyethylene-type polymer rather than apolypropylene-type. As will be seen hereinafter, as the highermethylene/methylidyne carbon atom ratio is approached, degradation isstill effected but the dosage requirements are more narrowly restrictedto the lower dosage range in order to avoid a resulting irradiatedpolymer exhibiting a major cross-linked portion.

Several methods of preparing a-olefin polymers are known in the art, asfor example, the processes relating to the preparation of polypropylenedescribed in Australian patent application No. 6365/55 to PhillipsPetroleum Company, United States Patent No. 2,692,259 to Edwin F. Petersand United States Patent No. 2,791,576 to Edmund Field. Interpolymers aswell as block copolymers of u-olefins with other olefinic and vinylmonomers such as ethylene and styrene, particularly those copolymerscontaining a major proportion of an oc-Olefin polymerized therein, arealso effectively employed. The a-olefins interpolymerized in saidcopolymers are preferably present in an amount of at least 50 percent byweight and most prefby weight.

The term ix-olefin polymer as used herein is intended therefore, toinclude such interpolymers and block co- Patented Oct. 214, 1967polymers as well as a-olefin homopolymers as set forth hereinabove.

The a-olefin polymer or resin compositions canalso include conventionaladditives such as colorants, stabilizers, lubricants, slip agents,plasticizers, fillers, and the like, and can be admixed with otherpolymeric materials either compatible or incompatible therewith.

The improved lX-OlefiH polymers of this invention can be produced bysubjecting them to ionizing radiation. As used herein, the term ionizingradiation includes that radiation which has sufficient energy to causeelectronic excita tion and/or ionization in the tat-olefin polymermolecules but which doesnot have sufiicient energy to affect the nucleiof the constituent atoms. Convenient sources of suitable ionizingradiation are high energy electrons produced by such means as Van deGraaff accelerators, linear electron accelerators, resonancetransformers and the like, gamma ray producing radioactive isotopes suchas Co and C5 spent nuclear fuel elements, X-rays such as those producedby conventional X-ray machines and the like.

Suitable ionizing radiation for use in the present invention generallyhas an energy level in the range of from about 50 kev. (kilo electronvolts) to about 20 mev. (million electron volts).

The predominant effect of ionizing radiation on a-olefin polymers hashereto-fore been considered to be the formation of a cross-linkedstructure. Moreover, the resuzting properties of the cross-linkedpolymer have been generally considered desirable whereas chain scissionresulting from irradiation has been considered detrimental and anyweakening or embrittlement of the polymer has been attributed thereto.Furthermore, a large degree of oxidation has also been associated withchain scissionresulting in rancidity and concomitant unpleasant odor andcolor rendering the polymer unfit for such commercial purposes as foodwrappings and the like.

Surprisingly, however, it has been found in this invention thatsubjecting m-olefin polymers in contact with air to ionizing radiationhaving an energy level in the range of about 50 kev. to about 20 mev. toa total dose of between about 0.01 to about 3 megareps but below thatamount which causes gelation provides a method for controllablydegrading wolefin polymer to any desired molecular weight level. Theincorporation of small amounts of antioxidant into the polymer prior toirradiation has been found, quite unexpectedly, to enable thedegradation to proceed atcontrolled rates. For example, a virginisotactic polypropylene, i.e., a freshly polymerized polypropylenehaving no antioxidant or other additive contained therein can bedegraded to a hard wax with a mere 1 megarep exposure to ionizingradiation (see Table II presented hereinbelow); whereas, isotacticpolypropylene containing a small amount of antioxidant, viz., less thanabout 0.5% of an antioxidant such as dibutyl para cresol or the like,when subjected to a similar 1 megarep dose is de-.

graded to a melt flow of about 19 dgm./min. (see Table III presentedhereinbelow) It is believed that the effect of the small amount ofantioxidant present in the polymer mass prior to irradia-. tion has theeffect of repressing oxidation while still enabling degradation tooccur. Thus, in a preferred embodiment of the present invention,a-olefin polymers having the repeating unit wherein R is an alkyl groupcontaining from about 1 to 16 carbon atoms and which contains astabilizing amount'of antioxidant, preferably about 0.01 to about 0.1percent by weight antioxidant, are uniformly and controllably degradedby subjecting said a-olefin polymer in air to ionizing radiation havingan energy level in the range of from about 50 kev. to about 20 mev. to atotal dose between mechanical shear degradation or pyrolysis. It isextremely difficult to mechanically shear all portions of a resin massabout 0.01 to about 3 megareps and preferably to a total dose of 0.1 to2 megareps. A megarep is equal to 1 10 reps; a rep being equivalent tothe absorption of 83.8 ergs per gram in the material subjected toirradiation.

Antioxidants which are suitable for use in the present invention arethose which can normally be employed in a-olefin resinssuch as propylenepolymers to stabilize such resins against oxidation. Exemplary of suchantioxidants are dibutyl para cresol, p-cresohformaldehyde resins,paratertiaryalkylphenol formaldehyde resins in admixturewith aminodithioformates, aliphatic polyepoxides, organic phosphites,thiopbosphates, or dithiophosphites, paratertiaryalkylphenolformaldehyde resins in admixture with mercapto compounds,2thiono-2-mercaptodioxaphosphorinane compounds,tetraphenylsuccinodinitriles or triphenylmethane, or dithiophosphatemetal salts and the like.

The radiation degraded ot-olefin polymers of this invention are found topossess unique melt rheological properties compared to a-olefin polymersdegraded by conventional pyrolytic and/ or mechanical shear methods.Quite unexpectedly, it has been found that radiation degraded a-olefinpolymers such as polypropylene can be extruded and drawn down at. muchhigher linear speeds without the occurrence of draw resonance orsurging, i.e., the phenomenon characteristic of thermoplastic extrusionprocesses which occurs at high linear speeds and causes uneventhicknesses or ripples. in the film or coating being produced.

High linear processing speeds are economically attractive and permitthinner films and coatings to be attained provided the drawdowncharacteristics of the resin permit such speeds. The higher no-surgespeeds, i.e., speeds at-. tained without causing uneven thicknesses orripples in the extrudate, attained with the radiation degraded a-olefinpolymers of the present invention are believed dueto a combination of.processes which occur when the a-olefin polymer is subjected to theextremely low doses of high energy ionizing radiation as discussedabove. It has been found that the major elfect of such dosages of highenerg radiation absorption by a-olefin polymers is, qute unexpectedly,chain scission although a definite but minor amount of cross-linkingalso takes place. The term crosslinking is employed herein to describethe formation of level, said level being a function of the molecularweight of the resin, the resin foms a three-dimensional gel network.This point is called the incipient gel point. Within the scope of themethod of the present invention the incipient gel point is not attaineddue to the predominating effectof chain scission. Accordingly, theirradiated a-olefin polymers of the present invention are essentiallygelfree. The net result is a different and unusual molecular weightdistribution which produces a .higher and more uniform draw-down rate athigh melt extrusion speeds without surging than can be obtained bypyrolysis or mechanical shear degradation.

While not wishing to be bound by any explanation of the theories ormechanisms'involved, it is believed that the results obtained can beascribed to the more thorough and uniform treatment of every portion ofthe resin mass by the high energy ionizing radiation as compared towithout producing extensive degradation of some parts of the resin.Also, pyrolysis is subject to heat transfer problems within. the mass ofmolten polymer which results in'non-uniform degradation with the moltenpolymer,

mass. In the radiation process herein described, each molecule of resinis surrounded by a cloud of high energy particles so that no portion ofthe polymer is able to escape treatment.

Due to the uniform degradation obtained by the method of the presentinvention, new and improved or-olefin polymers are obtained. Thesepolymers are unusual in that they are largely comprised of small sizedcrystallites due to the predominance of chain scission and of smallamount of cross-linking insufficient to cause gelation Moreover, it hasbeen found that the rate of crystallization of the irradiated a-olefinpolymers is significantly increased. For example, the rate ofcrystallization of an irradiated polypropylene of the present inventionis about twice that of a conventionally degraded polypropylene.

Typical freezing rate characteristics of the irradiated polypropylene ofthe present invention obtained with the aid of an extremely sensitivevolume dilatometer are compared with a polypropylene degraded to thesame melt index by conventional pyrolysis in Table I presented below:

TABLE I Pyrolysis Irradiated degraded polypropylene polypropylene (MI24) Initiation Range, C 8 6 Freezing temperature, C 138 140 MajorFreezing Rate, percent cryst linity/ O 12. 5 25 Major Freezing Range, C8 4 Overall Freezing Range, C 138-90 140-60 In general, the irradiatedmaterial has a broader overall freezing range but the major portion ofthe material freezes more rapidly, has a higher freezing point and asmaller initiation temperature diiferential than the untreated materialor that which has been degraded by pyrolysis or mechanical shear.Further analysis of the dilatometric data has shown that the irradiatedpolymers of the present invention have a substantially higherconcentration of small crystallites and a lower concentration of largecrystallites than the conventionally degraded polypropylene whichfurther indicates the greater uniformity of treatment obtained by themethod of the present invention.

Moreover, it has been found quite unexpectedly that the percentcrystallinity of the irradiated polymers of the present inventionincreases at a constant rate at temperatures between 0 C. and 100 C.whereas the same polymers prior to irradiation and those degraded bypyrolysis and/or mechanical shear have been found to exhibitaccelerating percent crystallinity within this range. The irradiatedpolymers of this invention are also unusual in that the percentcrystallite distribution of said polymers is constant at temperaturesbetween about 0 C. and 100 C. while the same polymers prior toirradiation or those degraded by other means have increasing percentcrystallite distribution within this range.

After irradiation, it is considered preferably to immediately blend theirradiated polymer with antioxidant and flux the mixture by compounding,extrusion or other thermal processing means at a temperature above themelting point of the resin but below that temperature at whichadditional degradation, as for example, through pyrolysis, would occur.After fluxing with antioxidant and other additives, if desired, thematerial can be pelletized or otherwise prepared for subsequent usage.

The following examples are merely illustrative of the present inventionand are not to be construed in any limitative manner. Unless otherwisespecified all percentages and parts are by weight.

Although the efliects of irradiation on a-olefin polymer can be observedin any thermoforming operation such as slot extrusion of sheeting orfilm, tubular extrusion of film, in pipe extrusion, monofilament andcontour extrusion of shaped moldings, rotational casting and the like,the examples presented hereinbelow are directed primarily to theextrusion coating of radiation degraded propylene polymers upon paper.It is clear, however, that the present invention is not to be consideredlimited either in scope or spirit to this embodiment.

Example I illustrates the unique tendency of propylene polymers tomarkedly degrade in molecular weight when exposed to very low doses ofionizing radiation whereas the other olefin polymers were notappreciably affected. As can be seen in Table II, it is consideredimportant that the propylene polymers, in particular, have in initialtensile modulus as measured by ASTM D638-T of at least 20,000 p.s.i. andpreferably above about 50,000 p.s.i. Below 20,000 p.s.i., the propylenepolymers are generally amorphous and rubbery and show essentially noresponse to the radiation dosages encompassed by the present invention.At higher radiation dosages, these amorphous species tend to cross-linkrather than degrade. The upper limit of tensile modulus is limitedsolely by the contemplated end use requirements.

Example 1 as follows:

TABLE II.EFFECT OF LOW DOSES OF IONIZING RADIATION ON VARIOUS OLEFINICPOLYMERS Melt Flow After Various Doses of Radiation Tensile No RadiationExposure 0.1 Megarep Exposure 1.0 Megarep Exposure Sample Descriptionlgidll llfs 44 p.s.i. 440 p.s.i. 44 p.s.i. 440 p.s.i. 44 p.s.i. 440p.s.i.

190 C. 190 C. 190 C. 190 0. 190 C. 190 C.

0. 48 50. 5 0.46 40. 6 566 57.8 Ethylene/propylene copolymer 63% pro- 5057 6 47 49v 6 455 46 6 Pyle 3. 3 212 1. 96 506 1.88 325Ethylene/butadiene copolymer (5% butadiene). 40, 000 2. O5 92 220 0. 2200 220 C. 220 C 220 C. 220 C.

Polyethylene (0.954 derislty) 1. t 80, 000 0.00 0.814 0.00 0.9980.007 1. 55 Pl 0 eneinrinsic ii i y l tli ii"22 912? .i 200, 000 0. 11325. 2 1. 63 251 131 Liquid *No antioxidant present in the samples priorto irradiation.

7 Example II Example II illustrates the range of melt flow obtainablewith various grades of polypropylene in pellet and powder forms as aresult of irradiation to varying doses with 2 million electron voltsfrom a Van de Graaff accelerator. It can be clearly seen that theresponse, as measured by melt flow of the polymer, to irradiation isslower and thus more controllable as the resin blend is increasinglyladened with antioxidant and other additives. Thus the melt flow oramount of degradation can be increased as the amount of radiation dosageis increased without fear of excessive or uncontrollable degradationoccurring as in Table II. The same elfect, i.e., controllabledegradation is noted when the polymer is subjected to radiation inpellet form as opposed to particulate form, i.e., as a powder or flake.It is believed that the effect of pelletizing and/ or the presence ofantioxidant or other foreign molecules interfere in some manner withirradiation thereby necessitating larger doses to obtain the same meltflow in a controllable manner as when the virgin polymer is employed.

In a preferred embodiment of the present invention, a-olefin polymersand more preferably propylene polymers, in powder form, containing astabilizing amount of antioxidant are subjected to irradiation asdescribed above and thereafter blended and fluxed with an additionalstabilizing amount of antioxidant and other additives such as dyes,fillers, and the like and formed into pellets.

The results summarized in Table III were obtained in a manner similar tothat described in Example I.

TABLE III.-EFFEOT OF VARYING DOSE N MOLECU- LAR WEIGHT OF VARIOUS TYPESOF POLYPROPYLENE AS MEASURED BY MELT FLOW Radiation Melt Flow SampleDescription Dose, decigrams megareps min., 230 C,

44 p.s.i.

Isotactic polypropylene (powder form), 0 1. 88 intrinsic viscosity=3,density= 0.905, 0.1 2. 07 0.07% dibutyl para cresol. 0. 11. 51

Polypropylene (pellet form) containing 0 4. 09 dibutyl para cresol anddilauryl thiodi- 0. 1 4. 52 propionate melt flow at 230 C.=4, den 0.5 5.53 sity=0.905. 1. 0 7. 76

99% crystallinity polypropylene (powder 0 0.004 form) 0.1% dibutyl paracresol and 0.15% 0. 1 0. 187 1,3,5-tris(3,5-dialkyl4hydroxy benzyl)- 1.0 10.18 2,4,6-trialkyl benzene.

Polypropylene (pellet form) containing 0 2. 55 about 0.1%1,3,5-tris(3,5-dialkyl-4-hy- 0.1 2. 94 droxy benzyl)-2,4,6-tria1kylbenzene and 0. 5 4. 05 trace amounts of dilaurylthiodipropi- 1. 0 6. 13oiate, density=0.905, melt flow 230 Polypropylene (pellet form)containing 0 3.0

about 0.1% .1,3,5-tris(3,5-dia1kyl-4-hy- 0.1 3.39 droxy benzyl)2,4,6-trialkyl benzene and 0. 5 4. 46 dilaurylthiodipropionate, density:

0.905, melt flow 230 C.=3.0.

The propylene polymers resulting from radiation degradation are fullyequivalent in physical properties to propylene polymers of the same meltflow obtained by other means. This is shown in Example III.

Example III A quantity of isotactic polypropylene having a reducedviscosity of 6 was given radiation doses of 0.36 and 0.75 megarep bypassing the resin, in powder form, under the beamof a 2 million electronvolt Van de Graaflf accelera tor while resting on a stainless steelconveyor belt. The melt flow of the resulting material after irradiationand stabilization with antioxidants were 6 and 12 decigrams per minuterespectively. The samples were extruded through a slot die onto achill-roll and wound up as flat films is shown in Table IV, incomparison with a control sample of polypropylene degraded by mechanicalshear degradation.

TABLE IV.-PIIYSICAL PROPERTIES OF SLOT CAST POLYPROPYLENE FILM Sample No1 2 3 Radiation Dose, megareps 0.36 0. 0 Mel Flow at 230 C.,decigrams/mm" 6 12. 3 5. 3 Gloss, 45 Dull 88 87 88 SpecularTransmission, percent. 86 89 90 Tensile Strength, p.s.i.:.

Machine Direction 6,080 6, 230 5, 550 Transverse Direction 8, 750 5, 0145,110 Percent Elongation:

Example I V pounds of isotactic polypropylene having a melt flow at 230,C. and 44 psi. of 0.16 and containing 0.08% dibutyl para cresol wassubjected to a radiation dose of 0.2 megarep in the manner describedabove. The resulting product was compounded with antioxidants and asuitable buffer in an extruder having a die maintained at 215 C. and aband heater at 210 C. The extruder drive was run at 1800 rpm. Theresinformulation was 0.015% calcium stearate, 0.15% dibutyl para cresol, and0.15% dilaurylthiodipropionate, the remainder being irradiated propyleneresin.

Biaxially oriented film was prepared using the process described inFrench Patent 1,274,521. Unoriented tubing was made by extruding theabove described polymer through an annular dieequipped with an annularorifice.

The extruder was operated at 380400 F. with the die.

maintained at 420 F. The tubing was drawn away from the die at four feetper minute by means of driven squeeze rolls, sufiicient internal airpressure being applied to obtain tubing of 2% inches in flat width and18 mils in thickness.

The flattened tubing was biaxially oriented by being inflated and fedthrough a radiant heater at a feed rate of 2.7 feet per minute andwithdrawn therefrom at a rate of 16 feet per minute. The oriented tubingproduced was 12 inches in fiat width.

The film produced from this irradiated resin was fully equivalent tothat blown from polypropylene degraded by thermal means. The propertieslisted below are similar to those which one would obtain from propylenecontaining non-nucleatin'g agents.

Haze PercenL- 6.7

Secant modulus Tensile n h 512'}; 5388 E Percent (MD) 70 onga Percent(TD) 61 duced viscosity of six were irradiated in powder form to a doseof 0.75 megarep. This treated polymer was blended film. The physicalproperties evaluation of the resulting 7 with anti-oxidants andstabilizers by continuous tumbling and was pellitized after passingthrough the extruder. The treated resin was found to have a melt flow at230 C. of 11 decigrams per minute. This resin was used to coat 30# kraftpaper at 588 F. using an extruder with a 12-inch coat hanger type slotdie. Extrusion was at a nominal rate of 36 pounds per hour. Forcomparison the commercial sample of polypropylene having a melt flow of14 was also extruded under comparable conditions. The radiation degradedsample produced 0.9-mil coating thickness at essentially the same nosurge speed and the melt flow 14 produced 0.7-mil coating thickness.

Example VI describes an experiment wherein isotactic polypropylene wasdegraded in molecular weight by radiation to a melt flow of 24 decigramsat 230 C. and extruded onto kraft paper at high speed. No surge speedsof 350 feet per minute with coating thickness of 0.3 mil were realized.A commercial sample of polypropylene having a melt flow of 23 decigramsper minute could n6t be extruded above 200 feet per minute withoutsurging the thinnest coating obtained was 0.6 mil. Another commercialsample of polypropylene having a melt flow of 31 decigrams per minutecould not be extruded above 250 feet per minute without surging. Thislatter sample which was produced by thermal degradation exhibited poordraw-down due to its low molecular weight.

Example VI A 100-pound sample of polypropylene having a nominal reducedviscosity of 3 was irradiated to a dose of 0.5 megarep with 2 millionelectron volts from a SOD-watt Van de Graaif accelerator equipped with a16-inch scanning width. The resin had a melt flow of 24 decigrams perminute. The resin was subsequently compounded with anti-oxidants andpelletized. This resin was used to coat 30# kraft paper using anextruder with a 12-inch coat hanger type slot die. Extrusion was at anominal rate of 36 pounds per hour. Comparison of extrusion coatingcharacteristics between commercial polypropylenes degraded by thermaland mechanical means and polypropylenes degraded by irradiation inaccordance with the present invention is shown in Table V.

TABLE V.EXTRUSION COATING DATA Sample 1 2 3 4 5 Radiation Dosage,

megarep 0.75 0 0. 0 Melt Flow at 230 C., 44

p.s.i l4. 0 l0. 9 22. 8 24 31. 2 Compound Temperature,

F 594 599 558 580 554 N0 Surge Speed, f.p.m 180 170 200 350 250 Neck-in, No Surge Speed,

in 2% 3% 2% 3% 3% Neck-in, 1.5 mil, in 2% 3 2% 2% 2% Basis Weight, 1.5mil, lbs./

rm 22 24 18. 8 Basis Wei ht, No Sur e Spee 10 13 8. 4 4. 0 6. 4 CoatingThickness, 1.5 mil,

mi 1. 6 1. 7 1. 4 Coatin Thickness, N0

SurgESpeed 0.7 0.9 0. 6 0.3 0.5

Example VII 0.15% dilaurylthiodipropionate and pelletized. The pelletswere extrusion coated onto 30# kraft paper using a compoundingtemperature of 305 C. The resin feed rate was 36 pounds per hour. Thedraw-down rate was 350 feet per minute without surging. A control sampleof polypropylene having a melt fiow of 24 gave a draw-down of 160 feetper minute without surging at the same feed rate and compoundingtemperature.

Example VIII An interpolymer of propylene and ethylene containing from 1to 2% ethylene combined was made using diethylene aluminum chloride andpurple TiCl catalyst. The birefrigent melting point of the sample was154 C. and the melt flow at 230 C. was zero. This polymer was irradiatedto a dose of 1.5 megareps. The resulting polymer had a melt flow at 230C. of 0.14.

Example IX A block copolymer of propylene and about 1 to 3 percent byweight ethylene was prepared using a diethyl aluminum chloride andpurple titanium trichloride catalyst system. Propylene was added to thereaction system at atmospheric pressure and ambient temperature for onehour followed by a nitrogen purge for about 30 minutes. Thereafter,ethylene was added for two minutes followed by a nitrogen purge for 15minutes. Finally, propylene was again added for one hour. The completecycle was again repeated and the resulting polymer was thereafterrecovered. With this catalyst system, the polymer chains are susceptibleto continuous growth throughout the reaction, hence, they are commonlyreferred to as living polymers, and their structure is substantiallydependent upon the duration of the particular monomer addition period.The resulting polymer contained infrared bands at 13.7 and 13.9 micronsindicating the presence of ethylene blocks. In addition, the spectrumshowed that about 97% of the polymer was highly crystallinepolypropylene in structure. The birefrigent melting point was 173.5 C.The melt flow of this polymer was 0 at 230 C. This sample was irradiatedwith 2 megareps dose of 2 million volt electrons. The resulting producthad a melt flow of 0.39.

Example X An ethylene-propylene block copolymer was prepared having tWopropylene blocks on either side of an ethylenepropylene copolymer block.The catalyst was the same as used in the Examples VIII and IX. Thefollowing sequence of polymerization was used. First propylene waspassed into a heptane-catalyst solution alone for 2 hours. Then ethyleneand propylene in a 1:1 volume ratio were added for 5 minutes. Then theethylene flow was discontinued and propylene flow continued for 2 hours.This polymer contained infrared bands characteristic of polypropylene aswell as bands at 13.7 and 13.9 characteristic of ethylene polymer. Thepolymer contained about polypropylene and had a melt flow at 230 C. ofzero. After a 1.5 megarep dose of 2 mev., the polymer had a melt flow of0.2 at 230 C.

Example XI An ethylene-propylene block copolymer consisting of one shortblock of ethylene and one long block of propylene segments was preparedusing the catalyst system employed in Example X by passing ethylene intothe catalyst system under atmospheric pressure and ambient temperaturefor 15 minutes followed by a nitrogen purge for 25 minutes. Thereafter,propylene was passed into said system for 5 hours at 20 p.s.i. Theresulting polymer was recovered and found to contain about 5% by weightpolyethylene and had a melt flow at 230 C. of 0.07. This polymer'wasstabilized with dibutyl para cresol and 1,3,5- tris (3,5 dialkyl 4hydroxybenzyl) 2,4,6 trialkyl benzene and irradiated with 2 mev. to adose of 2 1 1 megareps. The final melt flow was 4.5 decigrams/min. Theproperties of the final product were found to be:

1% tensile modulus p.s.i 150,000 Tensile strength p.s.i 5,000 Elongationpercent 1,000 Impact strength ft. lb./in. 40-60 This product was toostiff to fabricate prior to irradiation.

Example XII Properties:

1%tensile modulus p.s.i 130,000 Tensile strength p.s.i 4,800 Elongationpercent 1,000 Impact strength ft. lb./in. 30-50 A high level ofantioxidant concentration caused the radiation dose to be unnecessarilyhigh compared to earlier examples. The block copolymers can be similarlyaffected.

This product was too stiff to fabricate prior to irradiation.

Example XIII A block copolymer consisting of two long sequences ofpropylene chain segments on either side of a short randomethylene-propylene block was prepared employing the catalystsystemdescribed in the preceding examples. by passing propylene at 20p.s.i. into said catalyst systems for 2 hours and 35 minutes.Thereafter, the pressure was reduced to atmospheric and both propyleneand ethylene in a 1:1 volume ratio were passed into the system for 15minutes. The ethylene flow was then discontinued however, the propyleneflow was maintained for an additional 15 minutes followed by a finalcharge of propylene into the system at 20 psi. over a period of 2 hoursand 15 minutes. The resulting polymer was recovered and stabilized as inExample XII and was found to have a melt flow at 230 C. of 0.03dgm./min. A 2 megarep dose raised the melt flow to 3.8 dgm./min..

This product was too stiff to fabricate by compression molding prior toirradiation.

Example XIV A block copolymer of ethylene and propylene consisting of along propylene block and a random ethylene/ propylene copolymer blockwas prepared employing the catalyst system described hereinabove bypassing propylene and ethylene in a 1:1 volume ratio into the catalystsystem under atmospheric pressure for 15 minutes. Thereafter, theethylene flow was terminated and the propylene flow continued for anadditional, 18 minutes. Finally, the propylene was charged to the systemat 20 p.s.i. for 4 hours and 50 minutes. The resulting polymer wasrecovered and stabilized in the same manner as described in Example X11and was found to have a melt flow at 230 C. of 0.06 dgm./min. After 2.25megarep-s dose the melt fiow was 8.5 dgm./min.

1% tensile modulus p.s.i 120,000 Tensile strength ..p.s.i.. 6,000Percent elongation 1,200 Pendulum impact ft. lb./in. 30-50 12 Thispolymer was too stiff to fabricate into film by compression moldingprior to irradiation.

Example XV Table VI presented below illustrates the applicability of thepresent invention to the controllable degradation of the higher alphaolefin polymers. The flow properties, shown as either melt flow orreduced viscosity, are a measure of the net changes in molecular weightwhich result from chain scission and crosslinkage; an increase in meltflow is indicative of a decrease in molecular weight whereas an increasein reduced viscosity is indicative of an increase in molecular weight.Unless otherwise specified, the melt flow data was obtained at 190 C.and 44 p.s.i.. and the reduced viscosity data was obtained with samplesof 0.1 gram of the polymer dissolved in ml. of decalin at C. The'sampleswere subjected to irradiation in the manner hereinabove described.

Since p0ly(4-methyl-pentene-1) melts at 250 0., the melt flow data wasobtainedat 265 C.

to crosslink upon being subjectedto a dosage of two megareps indicatingthe low dosage crtiicality which exists at the higher limits of themethylene/methylidyne ratio, i.e., 16:1 as discussed hereinabove. It wasalso noted that poly(4-methyl-pentene-1), although having a repeatingunit containing six carbon atoms, behaved like polypropylene rather thanpoly(hexene-l). This is apparently due,

to the 1/ 1 methylene/methylidyne ratio possessed by bothpoly(4-methyl-pentene-1)1 and polypropylene as compared to the 4/1 ratioof poly(hexene-1).

The irradiated a-Olefil'l polymers of the present invention, due totheir unique rheological properties, have been found extremely useful inextrusion coating, slot and tubular extrusion of film, pipe extrusion,contour extrusion of shaped moldings, rotational casting and othersimilar thermoforming operations.

What is claimed is:

1. Method for uniformly and controllably degrading a-olefin polymerscontaining at least about 50 percent by weight a-olefin, the balancebeing an olefinic hydrohighest carbon copolymerizable therewith andhaving the repeating unit CHOH2 I. wherein R is an alkyl groupcontaining from about 1 to 16 carbon atoms which comprises subjectingsuch u-olefin polymer to ionizing radiation having an energy in therange of from about 50 kev. to about 20 mev. to a total dose betweenabout 0.01 to about 3 megareps, whereby the molecular weight of saidpolymer is decreased.

2. Method described in claim 1 wherein the a-olefin polymers are thosewhich possess a secondary carbon atom to tertiary carbon atom ratio offrom about 1:1 to 16:1.

3. Method described in claim 1 wherein the a-olefin polymers are thosewhich possess a secondary carbon atom to tertiary carbon atom ratio offrom about 1:1 to 6:1.

4. Method for uniformly and controllably degrading a-olefin polymerscontaining at least about 50 percent by weight wolefin, the balancebeing an olefin hydrocarbon copolymerizable therewith and having therepeating OH-CH2 ll wherein R is an alkyl group containing from about 1to 16 carbon atoms which comprises subjecting such a-olefin polymerscontaining a stabilizing amount of antioxidant, in air, to ionizingradiation having an energy level in the range of from about 50 kev. toabout 20 mev. to a total dose between about 0.01 to about 3 megareps,blending the irradiated polymer with a stabilizing amount of antioxidantand fluxing the mixture at a temperature above the melting point of theresin but below that temperature at which additional degradation canoccur, and thereafter recovering the irradiated, stabilized a-olefinpolymer.

5. Method for uniformly and controllably degrading a-olefin polymerscontaining at least about 50 percent by weight rat-olefin, the balancebeing an olefinic hydrocarbon copolymerizable therewith and having therepeating unit wherein R is an alkyl group containing from 1 to 16carbon atoms which comprises subjecting such a-olefin polymerscontaining less than about 0.5 percent antioxidant, in air, to ionizingradiation having an energy level in the range of from about 50 kev. toabout 20 mev. to a total dose between about 0.01 to about 3 megareps,blending the irradiated polymer with a stabilizing amount ofantioxidant, extruding the resulting mixture, and thereafter recoveringthe irradiated, stabilized a-olefin polymeric product.

6. Method for uniformly and controllably degrading a-olefin polymerscontaining at least about 50 percent by I 14 Weight ot-olefin, thebalance being an olefinic hydrocarbon copolymerizable therewith andhaving the repeating unit wherein R is an alkyl group containing from 1to 16 carbon atoms which comprises continuously introducing sucha-olefin polymers containing less than about 0.5 percent antioxidantinto a zone maintained at atmospheric pressure and ambient temperaturewherein said polymer is subjected to uniform exposure of ionizingradiation having an energy level in the range of from about 50 kev. toabout 20 mev. to a total dose between about 0.01 to about 3 megareps,and thereafter continuously recovering the irradiated product.

7. Method for uniformly and controllably degrading propylene polymerscontaining at least about 50 percent by weight propylene, the balancebeing an olefinic hydrocarbon copolymerizable therewith, said polymershaving an initial tensile modulus of at least about 20,000 p.s.i. whichcomprises continuously introducing such propylene polymers containingless than about 0.5 percent antioxidant into a zone maintained atatmospheric pressure and ambient temperature wherein said propylenepolymer is subjected to uniform exposure of ionizing radiation having anenergy level in the range of from about 50 kev. to about 20 mev. to atotal dose between about 0.01 to about 3 megareps, continuously removingthe irradiated propylene polymer and blending a stabilizing amount ofantioxidant therewith, fluxing the resulting mixture, and thereafterrecovering the irradiated, stabilized propylene polymer.

8. Method as defined in claim 7 wherein the propylene polymer ispolypropylene.

9. Method for uniformly and controllably degrading a-olefin polymers asdefined in claim 1 wherein the total dose of ionizing radiation isbetween about 0.01 and about 2 megareps.

10. Method for uniformly and controllably degrading propylene polymersas defined in claim 7 wherein the total dose of ionizing radiationisbetween about 0.01 and about 2 megareps.

References Cited Miller et al.: Journal of Poly. Sci., vol. 14, p. 503(1954).

Black and Lyons: Nature, vol. 180, 1346 (1957).

Charlesby & Pinner: Proc. Roy. Soc. (London), vol. A249, p. 367 (1959).

Waddington et al.: J. of Poly Sci., vol. 31, p. 221 (1958).

Chapiro: Radiation Chemistry of Polymeric Systems, Interscieuce Pub.1962, pp. 442446.

SAMUEL H. BLECH, Primary Examiner. MURRAY TILLMAN, Examiner. N. OBLON,R. B. TURER, Assistant Examiners.

1. METHOD FOR UNIFORMLY AND CONTROLLABLY DEGRADING A-OLEFIN POLYMERSCONTAINING AT LEAST ABOUT 50 PERCENT BY WEIGHT A-OLEFIN, THE BALANCEBEING AN OLEFIN HYDROCARBON COPOLYMERIZABLE THEREWITH AND HAVING THEREPEATING UNIT -(CH(-R)-CH2)WHEREIN R IS AN ALKYL GROUP CONTAINING FROMABOUT 1 TO 16 CARBON ATOMS WHICH COMPRISES SUBJECTING SUCH A-OLEFINPOLYMER TO IONIZING RADIATION HAVING AN ENERGY IN THE RANGE OF FROMABOUT 50 KEV. TO ABOUT 20 MEV. TO A TOTAL DOSE BETWEEN ABOUT 0.01 TOABOUT 3 MEGAREPS, WHEREBY THE MOLECULAR WEIGHT AND SAID POLYMER ISDECREASED.